This invention relates to a process for preparing N-phosphonomethylglycine by the simultaneous oxidation and dealkylation of an N-alkyl-N-(2-hydroxyethyl)aminomethylphosphonic acid. More particularly, the invention relates to a relatively simple method whereby N-phosphonomethylglycine is produced in high yield and yet relatively free of reaction by-products.
N-Phosphonomethylglycine, known also by its common name glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling a large variety of weeds and crops. It is applied to the foliage of a very broad spectrum of perennial and annual grasses and broad-leafed plants to achieve the desired control. Industrial uses include control of weeds along roadsides, waterways, transmission lines, in storage areas, and in other nonagricultural areas. Usually glyphosate is formulated into herbicidal compositions in the form of its various salts which retain the anionic form of glyphosate in solution, preferably in water.
Because of its commercial importance, many processes for making glyphosate have been published. One process for the manufacture of glyphosate is described by Gaertner in U.S. Pat. No. 3,927,080. Gaertner describes the production of glyphosate wherein N-t-butyl-N-phosphonomethylglycine or its esters are hydrolyzed under acidic conditions.
In European Patent No. 0,055,695, there is disclosed a process for splitting for a substituent group from the nitrogen atom of an N-substituted N-phosphonomethylglycine by catalytic hydrogenolysis. The N-substituent is described as a 1-arylalkyl group suitable for hydrogenolytic cleavage. The hydrogenolytic process is carried out in the presence of a catalyst, such as platinum or palladium on barium sulfate. The chemistry of the carbon/nitrogen bond of amines has been the subject of extensive study in recent years. For example, Murahashi and Watanabe disclosed the metal catalyzed reaction of tertiary amines with water in an article entitled "Palladium Catalyzed Hydrolysis of Tertiary Amines with Water" published in the Journal of the American Chemical Society, 101, 7429 (1979). In this publication it was reported that catalytic oxidation of tertiary amines proceeded generally and efficiently with palladium catalysts to provide secondary amines and carbonyl compounds.
Another process for the manufacture of glyphosate is described by Hershman in U.S. Pat. No. 3,969,398. In said process N-phosphonomethyliminodiacetic acid is catalystically oxidized to produce glyphosate.
A process to produce glyphosate using a metal catalyst is described in U.S. Pat. No. 4,442,041. This patent teaches a process for the conversion of the diethyl ester of [bis(2-hydroxyethyl)amino]methylphosphonic acid into N-phosphonomethylglycine in the presence of catalysts such as zinc oxide or cadmium oxide. The process described comprises the steps of:
(1) reacting the diethyl ester [bis(2-hydroxyethyl)amino]methylphosphonic acid in an oxygen free atmosphere with an alkali metal hydroxide selected from the group consisting of sodium hydroxide and potassium hydroxide, in the presence of water as solvent and a catalyst selected from the group consisting of zinc oxide and cadmium oxide at a high temperature and pressure for a sufficient period of time to cause the reaction to go to completion, and, PA0 (2) acidifying the product formed.
The process results in only a 33.1% yield of glyphosate in the one example described. This poor yield appears to be largely due to the formation of by-products by competing reactions. Oxidation of both 2-hydroxyethyl side chains would give N-phosphonomethyliminodiacetic acid (NP-IDA). In our studies this indeed seems to be the dominant component of the mixture produced by the process of the prior art as illustrated in example 12 of this specification. On the other hand dealkylation of both these side chains would yield aminomethylphosphonic acid. By contrast, the production of glyphosate requires the dealkylation of one side chain along with the oxidation of the other. Thus the yield of glyphosate appears to depend on the balance between the competing reactions.
A process has now been discovered by which substituted or unsubstituted N-alkyl groups can be cleared from N-alkyl-(2-hydroxyethyl)aminomethylphosphonic acids (hereinafter called NNAMP acids) while the 2-hydroxyethyl group is simultaneously oxidized under alkaline conditions in the absence of any catalyst.
In surprising contrast to the teaching of the U.S. Pat. No. 4,442,041 it has now been shown that if the heavy metal catalyst described is omitted, the yield of glyphosate is considerably increased over that described, even when the reaction is operated under otherwise equivalent conditions. Example 12 shows a detailed examination of the disodium salt of [bis(2-hydroxyethyl)-amino]methylphosphonic acid under catalytic and non-catalytic conditions and demonstrates the substantial increase in glyphosate yields under the non-catalytic conditions. The uncatalyzed process was found to give consistent yields of N-phosphonomethylglycine in excess of 50%, as opposed to the maximum 33% yield of N-phosphonomethylglycine reported in U.S. Pat. No. 4,442,041.
In addition to the improved yields when no catalyst is used, there is also a distinct environmental (as well as economic) advantage in not using such catalysts. In such a reaction some catalyst is inevitably entrained with the waste stream. The complete removal of such heavy metals is not easily accomplished and they can often find their way into the environment as pollutants.